The history of ancient Israel can be summed up in five depressing steps: 1. The Israelites turned away from God. 2. God punished the Israelites, usually by sending a foreign nation to attack them. 3. The Israelites turned back to God, and called on him for deliverance. 4. God rescued the Israelites from their enemies. 5. The Israelites went back to step 1, and the process repeated itself. In this passage, Joshua calls the Israelites out on this, saying "you are not able to serve the Lord".
There are two things to take note of here, though. Firstly, we are a lot like the people of ancient Israel; no matter how sincere we are about committing (or re-committing) our lives to the Lord, sooner or later our devotion dies down and we wander away. Secondly, now matter how often the Israelites turned away, God always called them back to him, and always recieved them when they did turn back.
God is always ready and willing to accept people who turn back to him, no matter how often they have turned away from him in the past. We need not fall into despair when we find ourselves wandering away from God. Instead, we should take comfort in the fact that our salvation depends not on our own efforts, but on Jesus Christ, who remained fully devoted to God his Father, even to death.
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